Knit 15 sts, BO 31, knit the rest of the sts, which with the leftover loop from the bind off will also have 15 sts. You need to start the BO with another strand of yarn, either from another skein or the other end of the one you did the first shoulder with.
At the end of the knit row where you bound off, turn and purl back, dropping the one yarn when you get to the neck back, pick up the other end and BO 3 sts then finish the row. Turn and knit to the neck, drop the yarn, pick up the other one and BO 3 sts and knit the rest. On the next WS row, do the same, except you BO 2 sts when you get to that shoulder, and also on the RS row after that. When your armhole measures 8 or 8½" BO at the outer edge at the beginning of the rows.
sue- knitting heretic